Over the past decade the use of rotary hydrodynamic blood pumps for both short term and long term circulatory support has expanded. The longest survival by any patient with a single heart assist device at the time of this application is seven years and ongoing utilizing a Jarvik 2000 axial flow left ventricular assist device with blood immersed ceramic bearings. In more than 200 cases using the Jarvik 2000 heart no mechanical bearing has ever failed due to wear or fatigue fracture. Other models of ventricular assist devices utilizing blood immersed ceramic bearings have also proven durable, such as the HeartMate II VAD, which is largely patterned after the Jarvik 2000. Computer analysis of the Jarvik 2000 bearing design supports the expectation that durability of 10-20 years or more is likely to be achieved.
In any rotating pump, a spinning impeller imparts hydrodynamic energy to the fluid. All rotational pumps must contain three elements, a housing which contains the fluid, a rotor which spins within the housing and imparts energy to the fluid, and a bearing system which supports the rotor, permitting rotation. Additionally, pumps require means to transmit torque to the rotor, which may be via a sealed shaft, or by magnetic forces.
Rotary blood pumps using a wide variety elements to achieve these requirements have been disclosed, and the rotary blood pump prior art now includes hundreds of patents. Generally, three types of bearing mechanisms are employed, 1) mechanical bearings, using fluid film lubrication or hydrodynamic fluid support, 2) fluid levitation, and 3) magnetic levitation. Also, various combinations of these are employed for thrust and radial support.
The present invention deals principally with mechanical blood immersed bearings in which fluid film lubrication is employed to support the radial bearing load, and complete, or partial magnetic thrust load support may additionally be provided.
In addition to providing a highly reliable and durable means of supporting the rotor, bearing systems for blood pumps must be hemo-compatible causing little blood damage or thrombus formation. In the case of mechanical bearings, this is generally accomplished by high flow washing of the junction of the rotating and stationary parts of the bearings as claimed in my early U.S. patent (Jarvik—U.S. Pat. No. 4,994,078). The present invention is an improvement over '078, providing an important new structure to achieve better blood flow washing of the bearings.
Usually axial flow blood pumps incorporating mechanical bearings use a tapered rotor having larger diameter in its center and tapered to a small diameter at each end. This permits small diameter bearings to be used, which is advantageous because the friction at the bearing surfaces is reduced compared to larger diameter bearings; this limits wear, power consumption by the bearing, and heat generation. Examples from the prior art include small ball in cup bearings, Burgreen—U.S. Pat. No. 6,093,001), jewel bearings with an olive and endstone, (Benkowski—U.S. Pat. No. 5,947,892), sleeve bearings with conical thrust bearing surfaces (Jarvik—U.S. Pat. No. 5,613,935) or with flat thrust bearing surfaces (Bozeman—U.S. Pat. No. 5,692,882), and grooved conical hydrodynamic bearings (Carrier—Pub No. 2007/0004959) adapted to carry both thrust and radial loads.
Another approach to mechanical blood immersed bearings disclosed in the prior art is lubricated support on the tips of pump impeller blades, or on shrouds surrounding impeller blades as disclosed by Shambaugh in U.S. Patent Application Publication No. 2007/0078293, which discloses a rotor supported on wide blade tips having a cylindrical portion and a tapered portion to support thrust loads. In U.S. Pat. No. 5,211,546 FIG. 7A, Isaacson disclosed an impeller hydro-dynamically supported on a shroud supported on the tips of the impeller blades. Both of these configurations have the disadvantage that the viscous friction in the gap between the rotating impellors and stationary housing is high, since the rotational velocity of the blade tips or shroud is maximum at the tip diameter.
Blood pumps using full magnetic levitation can be designed such that the dimension of the gaps between the magnetically levitated rotor and the stationary housing is large enough that blood damage or power losses due to shear in the gaps is negligible. The disadvantage of fully magnetically levitated blood pumps is that they are relatively large and complex compared to miniature pumps using mechanical bearings. In addition to the larger size and weight, they require active electromagnetic feedback control to maintain stability of the rotor, and this presents reliability issues with the electronics. If magnetic support is lost, the rotor will “crash” into the housing or stator blades, which could cause damage. Antaki, U.S. Pat. No. 6,761,532 provides wear resistant coatings on the blade tips, rotor hub, and housings of magnetically levitated blood pumps to reduce or eliminate damage in the event of failure of the magnetic support system. In FIG. 1, Antiki shows a structure where a tapered portion of the rotor hub, 38 can contact the tips of stator blades, 36, if the magnetic bearing system fails. The structure shown appears similar to some embodiments of the present invention, but would not provide a stable bearing system for the rotor, because the rotor would tilt from its usual rotational axis within the housing, and is not axially constrained.
In the bearing system of the present invention, the rotor of a hydrodynamic blood pump is rotationally supported on mating portions of the tips of support blades which contact its hub close to the center of rotation, and is axially restrained to prevent disengagement of the proper mating position of the rotating bearing surface in relation to the stationary bearing surfaces. In the preferred embodiment, two opposing bearings at each end of the rotor, limit axial and radial motion to as little as 50 millionths of an inch, while providing completely unconstrained rotational freedom.
The most important aspect of the present invention involves the pattern of washing of the bearings, by blood flow across them, to prevent thrombus. All other blood immersed mechanical bearing designs, except those that support the rotor on the impeller blade tips, (which has disadvantages described above) present a complete circumferential ring of bearing material to the bloodstream. This is an area of local flow stasis. Some thrombus tends to form on the surface. This then forms a continuous circumferential ring of thrombus surrounding the rotor adjacent to the junction of the rotating and stationary parts of the bearing. If high enough flow is provided across this area of the pump, the ring of thrombus may remain limited, in the form of a thin torus, and not become sufficiently large to interfere with the function of the pump. But if the material becomes infected, or if the flow is reduced to too low a level, or if the patient is hypercoagulable, the amount of thrombus may increase. The present invention eliminates any continuous ring of bearing material which can support growth of a torus of thrombus as described, while limiting the circumferential relative speeds of the bearing surfaces to the lowest practical values (because the bearing diameter is much less than the pump impeller tip diameter. This represents a major improvement over other blood immersed bearing designs.
A portion of the rotor surface having a small diameter relative to the impeller tip diameter is made of hard wear resistant material, and is circular on cross section at any point perpendicular to its axis of rotation. This surface may be cylindrical, conical, or another shape. The tips of two or more support posts, placed at generally uniform spacing around the circumference of the rotor and having mating surfaces in rotational contact with the bearing surface on the rotor, prevent the rotor from moving radially away from its axis of rotation. The contacting surfaces on the ends of the support posts constitute bearing “pads” upon which the bearing rotates. In the preferred embodiment there are two sets of these support posts, near each end of the rotor, and the bearing pads are tapered so that they act as both radial and thrust support members. The support posts are elongated and streamlined, and have the appearance of short blades. In a configuration with three such support blades at each end of the rotor, the spaces at the surface of the rotor between the blades are freely washed by generally axial flow proceeding through the pump.
Thus, in this region, formation of a torus of thrombus, which could enlarge and lead to problems, is avoided. A blood pump using this type of bearing can be designed to remain entirely free of thrombus accumulation throughout, as a fully magnetically levitated pump can also be. But the pump using the mechanical bearings of the present invention can be much simpler and much smaller.